Electroless plating of non-conductors



Feb. 3, 1959 P. H. EISENBERG 2,872,312

ELECTROLESS PLATING OF NON-CONDUCTORS Filed Jan. 2e, 195e E/efzra/efJ/y IN VEN TOR. UH/l. H. E /SE NBE R6 BY lim-m QQM.:

2,872,3l2 Patented Feb. 3, 1959 2,872,312 ELECrRoLEss PLATING or NoN-coNnUcToRs Philip H. Eisenberg, Hicksville, N. Y., assignor to Sylvania Electric Products Inc., a corporation of Massachusetts Application January 26, 1956, `Serial No. 561,599

` 2 Claims. (Cl. 75212) My invention is directed toward methods for producing metallized bodies.

The term metallized bodies refers to an electrically non-conductive or poorly conductive body formed for example from a ceramic, a semiconductor or a plastic having an exposed surface coated with metal. Such bodies, which I define as essentially electrically nonconductive bodies, are often used for example in forming vacuum tight ceramic to metal seals.

Conventionally, metal coatings or layers are formed on such bodies by first applying the coating to the body in the form of a powder or paste, and then ring the coated body at elevated temperatures (and often in the presence of an inert or non-oxidizing atmosphere).

In order to provide a less costly and complex procedure, it has been proposed to electrolytically deposit a suitable metal coating on such bodies. In order to deposit coatings in this manner, since such bodies do not have suicient conductivity to sustain an electrolytic deposition process, the body surfaces which are to be metallized must first be treated to render them electrically conductive, as for example by sensitizing these surfaces in a stannous chloride solution. Metal can then be electrolytically deposited onto the sensitized surfaces.

However, this electrolytic deposition method has not been particularly successful, because the deposited layer is not wellbonded to the body, and tends to chip, crack or break.

I have invented a process for depositing metal coatings on the surface of an essentially electrically nonconductive body which partakes all of the advantages of the electrolytic deposition process and yet further produces a deposited layer which is well bonded to the receiving surface. This process is also adapted to the production of cermets as will be explained in more de tal hereinafter.

Accordingly it is an object of the present invention to provide a new and improved process of the character indicated.

Another object is to improve methods of metallizing essentially electrically non-conductive bodies through the use of electroless deposition.

Still another object is to provide a process for depositing a metal coating onto the surface of an essentially electrically non-conductive body which can be carried out at temperatures below 100 C. and in the presence of air and yet which does not require any source of electrical energy.

Still a further object is to provide a new and improved method for making cermets.

These and other objects of my invention will either be explained or will become apparent hereinafter.

My invention contemplates the use of electroless plating'. Certain aspects of the electroless plating or deposition technique are known to the art. Other aspects of this technique are described in more detail in my copending applications Serial No. 561,648, filed January 26,

1956, now U. S. Patent No. 2,829,059, Serial No. 561,- 670, tiled January 26, 1956, now U. S. Patent No.

2,827,398, and Serial No. 561,560, led January 26, 1956,

now abandoned.

Briefly, however, electroless plating is a process for depositing at least one component selected from the group composed of iron, chromium, nickel, cobalt and the vanadium alloys thereof out of an aqueous electroless plating solution onto the surface of a metallic substraterby virtue of a catalytic action which proceeds without the use of an applied electric eld.

The plating solution normally contains the metallic component or components to be deposited in the form of a salt such as a chloride or sulphate, together with a hypophosphate salt of an alkaline earth metal and a buering or buffering and seqnestering agent which is an organic salt.

The parameters of this solution, such as the pH, temperature, and concentration ratio of the various solution constituents are maintained within predetermined limits to insure that the component is deposited only upon the surface of the substrate and, for example, does not deposit upon the inner walls of the vessel containing the solution.

When these parameters are not controlled in this manner, the component will be deposited inan erratic manner upon any surface in contact with the solution, whether or not this surface is electrically conductive. An electroless plating solution in which this type of action takes place is known to the art as an unbalanced electroless plating solution as compared to a conventional or balanced electroless plating solution.

In my invention a selected outer surface of an essentially electrically non-conductive body, such as a ceramic, a semiconductor, or a plastic, is slightly roughened, as, for example, by etching, to form minute surface irregularities or pores thereon. The body is then dipped into an'aqueous solution of a hypophosphite salt (for example a sodium hypophosphite solution) to saturate these surface pores. At this point, the body is removed from this solution and placed in a conventional electroless plating solution. The plating solution parameters are controlled in conventional manner. However, the plating solution in the region of the surface pores is unbalanced because of the localized hypophosphite concentration, and minute particles of the component or components to be deposited (that is at least one component selected from the group composed of nickel, cobalt, iron, chromium and the vanadium alloys thereof) are erratically deposited within these pores. These minute particles then act as nuclei or seeds about which the electroless deposition forms.

Alternatively, the body can rst be placed in an unbalanced solution to initiate the seeding action, and then the rbody can be removed from the unbalanced solution and placed in the conventional plating solution.

In either case, the pores are first impregnated with the deposited component and a surface coating of this component subsequently forms. This coating is mechanically keyed to the roughened surface to form a strong durable mechanical bond therewith.

I have found that unless the above described seeding action takes place, such a bond cannot be formed. However, if the selected surface of the essentially electrically non-conductive body exhibits inherent surface irregularities, the surface roughening treatment can be dispensed with. n

I have also discovered that when individual ceramic particles are coated with metal in this fashion, the coated particles can be formed into a cermet by the use of conventional powder metallurgy techniques. Such cermets are found to have an extremely uniform structure; that is the metallic phase is uniformly distributed about the ceramic particles.

My invention will now be described in detail in the examples which follow and with reference to the laccompanying figure which is a photo-micrograph of a metallized body produced in accordance with my invention.

Example I An aluminum oxide disc Was degreased in a conventional steel type or heavy duty cleaner. A selected surface ofthe cleaned disc was then etched in a sulphurichydrofluoric acid solution.

The etched surface of the disc was then dipped into an aqueous solution of sodium hypophosphite (150 Vgrams/ liter concentration) Which was maintained at a temperature of 90 C.

When the pores of the etched surface had absorbed the hypophosphite solution, the disc was then placed in an electroless plating solution having the following composition:

Grams/liter Nickel sulfate 30 Sodium acetate 10 Sodium hypophosphite 10 The plating solution was maintained at a temperature of Sly-90 C. and the solution pH was held within the range 4-6. An electroless nickel deposition then formed in conventional manner.

A photo-micrograph of a cross section of the plated disc is shown in the accompanying figure. This photograph clearly shows that the plating was initiated in the etched regions of the surface and then continued to coat the entire surface.

In order to test the mechanical bond between the nickel and the aluminum oxide disc, a number 16 copper wire was soldered to the nickel coating. The plating disc was then securely clamped and the copper wire was pulled perpendicularly to and away from the nickel coating. Ultimately, the disc itself cracked but the nickel coating remained bonded to the surface of the disc.

Example II Aluminum oxide particles (minus 325 mesh size) were nickel plated in the manner set forth in Example I. The same mechanical bond was formed between the nickel and the aluminum oxide particle.

Example III Example I was repeated using diierent bath temperatures and deposition rates (the rate at which coating thickness increases) were measured for each temperature as indicated in Table I below.

It was found that temperatures of 8085 C. represented optimum operating temperatures because the plating bath life decreased at higher temperatures.

4 Example 1V Nickel coated aluminum oxide particles produced as in Example II were loaded into a disc and cold pressed into a compact using a pressure within the range 5-10 tous per square inch. v

The compact was then sintered in an oxygen-hydrogen atmosphere, and at a temperature of 1200 C. for a period of 15 minutes.

It was found that a nickel-aluminum cermet was formed. A sectional analysis of the cermet revealed that the nickel was uniformly dispersed about the aluminum particles.

Example V Example l was repeated using an electroless plating bath containing sodium citrate in place of the sodium acetate. The pH of this solution was maintained within the range 8-10. All other plating parameters were maintained as in Example I.

It was found that the results were substantially identical to those of Example I.

While I have shown and pointed out my invention as applied above, it will be apparent to those skilled in the art that many modications can ybe made within the scope and sphere of my invention as defined in the claims which follow.

Whatlis claimed is:

1. A method for forming a cermet from ceramic particles having minute surface pores which comprises the steps of chemically depositing minute particles of at least one metallic component selected from the group consisting of iron, nickel, cobalt, chromium and the vanadium alloys thereof into said pores to seed said pores with said particles; electrolessly coating the ceramic particle surfaces with said component, said component impregnating said seeded pores and being mechanically keyed to said surfaces; and pressure forming and sintering said particles together to form a cermet, the said one component being uniformly dispersed about the ceramic phase of said cermet.

2. TheA method as set forth in claim l, wherein said seeding and coating steps are carried out substantially simultaneously.

References Cited in the file of this patent UNITED STATES PATENTS 2,439,654 Gaiser et al Apr. 13, 1948 2,454,610 Narcus Nov. 23., 1948 2,690,401 Gutzeit et al. Sept. 28, 1954 2,690,402 Crehan Sept. 28, 1954 2,690,403 Gutzeit et al Sept. 28, 1954 FOREIGN PATENTS 181,590 Switzerland Mar. 16, 1936 OTHER REFERENCES Protective Coatings for Metals (R. M. Burns and W. W. Bradley), Reinhold Publishing Corp., 2nd edition (1955); page 594, relied upon. 

1. A METHOD FOR FORMING A CERMET FROM CERAMIC PARTICLES HAVING MINUTE SURFACE PORES WHICH COMPRISES THE STEPS OF CHEMICALLY DEPOSITING MINUTE PARTICULES OF AT LEAST ONE METALLIC COMPONENT SELECTED FROM THE GROUP CONSISTING OF IRON, NICKEL, COBALT, CHROMIUM AND THE VANADIUM ALLOYS THEREOF INTO SAID PORES TO SEED SAID PORES WITH SAID PARTICLES; ELECTROLESSLY COATING THE CERAMIC PARTICLE SURFACES WITH SAID COMPONENT, SAID COMPENENT IMPREGNATING SAID SEEDED PORES AND BEING MECHANICALLY KEYED TO SAID SURFACES; AND PRESSURE FORMING AND SINTERING SAID PARTICLES TOGETHER TO FORM A CERMET, THE SAID ONE COMNONENT BEING UNIFORMLY DISPERSED ABOUT THE CERAMIC PHASE OF SAID CERMET. 